This invention relates to focus detecting devices, and more particularly to a focus detecting device which is applicable to cameras.
Focus detecting methods have been proposed in the art, in which focussing is detected by utilizing the fact that the contrast, spatial frequency or light quantity of an object is changed according to the degree of focussing. In a first of the conventional methods, a photoelectric element such as a CdS cell having a particular contrast sensing characteristic is utilized. In a second method, a photelectric output signal obtained by mechanically vibrating or rotating an optical member is utilized. In a third method, similar to the case of a range/view finder, two outputs of light detecting members disposed at different positions for detecting the focus condition.
However, the first method suffers from the disadvantage that it is difficult to detect correct focus with high accuracy. This is because it is difficult to manufacture photoelectric elements having the same light response characteristic. Also, the output variation rate of the photoelectic elements is low before and after focussing is obtained in detecting the focussing in situations of low intensity illumination. The second method is disadvantageous in that it is difficult to minaturize the camera because it has mechnaically movable components. Such a system also consumes relatively large quantities of electric power. The third method is also disadvantageous in that it is difficult to have interchangable lenses in range finder types of cameras.